The present invention concerns a process for preparing ferrous parts of a color television tube and a furnace for operating such a process.
In order to realize a suitable image, it is known to dispose inside the glass tube constituting the shell of the cathode tube, ferrous parts such as the magnetic shield, the shadow mask and its frame. According to this technology, the mask is mounted in a frame which is placed on the rear face of the screen. A first problem concerning the object of the invention relates to the natural deposit of rust on this kind of part during manufacturing process. Indeed, for reasons of cost, mechanical and electrical behavior, the frame-mask assembly is constituted by iron that becomes oxidized in a Fe.sub.2 O.sub.3 oxide. This oxide is formed on the surface of the ferrous part and spreads towards the core of the part by eroding it. There is thus a deterioration of the said part. Furthermore, slightly adhesive rust particles are thus formed which can be separated off the ferrous parts and disturb the correct operation of the tube.
Moreover, the tube comprises behind the base a cone ended by a glass neck that allows to obtain a vacuum sealed tube.
As it is known, the neck bears the electron guns and the magnetic deviation assembly. The cone is doubled inside by a magnetic shield constituted by a ferrous part that fits exactly the shape of the cone. This ferrous part allows on the one hand, to close the lines of the magnetic field emitted by the front of the deviation assembly (magnetic conduction), and on the other hand, to form a black body with the mask for the various radiations. The natural deposit of rust is also prejudicial.
A second problem concerning the object of the invention relates to the natural constitution of mechanical stresses induced in the metallic parts. These stresses must be cancelled out so that the shape of each part remains stable. An annealing treatment of the ferrous parts is thus necessary.